The present invention relates, in general, to the field of DC motor control circuits. More particularly, the present invention relates to an "H" switch circuit utilizing power MOSFETs for providing bidirectional control of fractional horsepower DC motors.
The DC low voltage fractional horsepower motor is a standard for applications that require bidirectional shaft control such as, cassette tape drives, automobile power windows, seats, door locks or windshield wipers, robotic mechanical controls, small industrial machines, etc. Heretofore, mechanical reversing gears and linkages have been used for providing such bidirectional control, particularly in automobile applications.
Previously, there has been described an "H" switch circuit for use in reversing the motor supply voltage for bidirectional control of a DC motor. Opposing top and bottom switches coupling the DC motor input leads to a voltage source and circuit ground control the motor direction by switching the voltage polarity, which in turn, switches the motor's shaft rotation either clockwise or counterclockwise. Heretofore, conventional bipolar devices have been used in such circuits. However, the bipolar design is not very practical because of the large base drive requirement of the transistors. The substitution of Darlington transistors in the "H" switch concept is also not practical for low voltage high current applications due to the large forward voltage drop across the devices which in turn reduces the voltage to the motor resulting in a slower operating motor. This is especially undesirable when dealing with a generally fixed voltage source such as an automobile battery and when optimum motor speed is essential. Further, although the bipolar devices would allow for direct pulse width modulation in switching, such that motor speed could be controlled, such devices are not readily interfaced with microprocessors for controlling motor speed. The Darlington devices, moreover, although capable of being directly driven, are undesirable due to their large forward voltage drop.